You could try:
Object.setimagetrigger(0,1);
Where, 0 is the mountpoint righthand, and 1 is on.
(not sure which kinds of objects support it, AIPlayer, Player, WheeledVehicle, and StaticShape I know that they support it.)
or:
WeaponImage::onFire(%this,%obj,%slot);
Where, %this is the image name (ex. SwordImage), %obj is the player, and %slot is the mountpoint.
Ex. WeaponImage::onFire(GunImage,%obj,0);